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The mean free path, in gases, is of the order of:
1. 10-10 m
2. Hundreds of angstroms
3. Thousands of angstroms
4. None of these

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Correct Answer - Option 3 : Thousands of angstroms


Molecular nature of matter:

  • From many observations, in recent times we now know that molecules (made up of one or more atoms) constitute matter.
  • Electron microscopes and scanning tunneling microscopes enable us to even see them.
  • The size of an atom is about an angstrom (10-10 m). In solids, which are tightly packed, atoms are spaced about a few angstroms apart.
    • In liquids, the separation between atoms is also about the same. In liquids, the atoms are not as rigidly fixed as in solids and can move around. This enables a liquid to flow.
    • In gases, the interatomic distances are in tens of angstroms.
  • The average distance a molecule can travel without colliding is called the mean free path.
    • The mean free path, in gases, is of the order of thousands of angstroms.
  • The atoms are much freer in gases and can travel long distances without colliding.
  • If they are not enclosed, gases disperse away.
  • In solids and liquids, the closeness makes the interatomic force important.
  • The interatomic force has a long-range attraction and a short-range repulsion. The atoms attract when they are at a few angstroms but repel when they come closer.
  • The gas is full of activity and the equilibrium is a dynamic one. In dynamic equilibrium, molecules collide and change their speeds during the collision. Only the average properties are constant.
  • Atoms consist of a nucleus and electrons. The nucleus itself is made up of protons and neutrons. The protons and neutrons are again made up of quarks.


  • The average distance a molecule can travel without colliding is called the mean free path.
  • The mean free path, in gases, is of the order of thousands of angstroms. Hence, option 3 is correct.

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